Drug resistance/disease relapse in the majority of T-cell acute lymphoblastic leukemia (T-ALL) patients demands better understanding T-ALL biology that may help develop more effective therapy. Mammalian target of rapamycin (mTOR), present in two cellular complexes: mTORC1 and mTORC2, has been related to tumorigenesis and/or drug resistance in several cancers. Targeting mTORC1 by rapamycin analogs has been tried in various tumor clinical contexts. However, the length of patient response is limited and not all patients respond. To overcome these limitations, the second generation of mTOR inhibitors have been developed to target both mTORC1 and mTORC2. This project is motivated by our new finding that mTOR functions upstream of FANCD2 of the Fanconi anemia DNA repair pathway to regulate DNA damage response (DDR) in T-ALL cells. The proposed studies focus on testing the novel hypothesis that mTOR-regulated DDR constitutes an important drug resistance machinery in drug-resistant/relapsed T-ALL, and targeting this pathway increases effectiveness of DNA damaging chemotherapy. The research will examine whether a mTOR-FANCD2-ATR-Chk1/ATM-Chk2 DDR signaling node exists and is hyper-activated in drug- resistant/relapsed T-ALL (Aim 1). The research will further determine whether targeting mTOR-regulated DDR pathway using dual mTORC1/mTORC2 inhibitor combined with DNA damaging drug overcomes drug resistance in mice xenografted with drug-resistant/relapsed primary T-ALL patient cells (Aim 2). Successful execution of the project will not only help us in innovating our understanding of drug resistance in T-ALL therapy, but also allow a long sought-after DNA damage sensitization therapeutic principle to tackle drug resistant T-ALL.